The present invention relates to a start system for a gas turbine engine, and more particularly to a dual ignition system that provides an environment conducive to igniting a gas turbine engine while providing diagnostic information.
Gas turbine engine ignition systems typically include an ignition exciter that generates the discharge energy used to fire the ignition plugs or igniters. These circuits are commonly used within the aviation industry, but are not limited to aircraft turbine engines. For example, gas turbine generators and other small turbine engines will typically utilize exciter circuits to provide spark energy to one or more igniter plugs.
A start sequence for a gas turbine engine coordinates engine speed, ignition and fuel delivery to achieve a reliable start. Conventionally, a dedicated starter motor or a starter-generator, is drivably coupled to the gas turbine engine and is operated to produce rotation thereof. As the starter accelerates the engine, a fuel delivery pump driven by a gearbox attached to a rotor of the gas turbine engine provides fuel flow thereto. The igniters are then fired to effect ignition in a combustor of the engine. Upon successful ignition, and once the engine has reached a self-sustaining speed, the starter is disengaged.
Gas turbine engines, specifically Auxiliary Power Units (APUs), often employ an ignition exciter box to drive two ignition plugs. The ignition plugs are positioned around the combustor, separated in position for optimal ignition of the air/fuel mixture. Each of the two ignition plugs may be driven independently. For optimum ignition, it is common to fire both ignition plugs together such that the fuel-air mixture is ignited at two locations to foster quick propagation of the flame.
Conventionally, exciter diagnostic circuits are designed for an ignition exciter to provide fault diagnostic data for each of the two ignition plugs and driver circuits. Another approach is to drive only one ignition circuit during a start sequence at a time, and use a failure to start as a diagnostic indication that an ignition circuit has failed. The problem with driving a single ignition circuit at a time is that lighting of the combustor in a single area by one ignition plug may not provide as reliable a chance of a successful engine light.
Accordingly, it is desirable to provide an ignition system for a gas turbine engine that ensures start reliability without dedicated electronic diagnostic circuits in an inexpensive, uncomplicated and lightweight arrangement.